Among the agents used in pain management, opioid analgesics are the most efficacious in controlling moderate and severe post-operative pain. However, opioid analgesics have well-known adverse effects such as respiratory depression, constipation, nausea, and neurotoxicity (Bruera et al., 1999), as well as tolerance and physical dependence. Decades of research have focused on designing an opioid analgesic that has the analgesic efficacy of morphine but is devoid of its adverse effects.
Pharmacological studies have defined three classes of opioid receptors, the δ, κ, and μ opioid receptors. One approach to develop agents having opioid analgesic efficacy in the absence of tolerance and/or physical dependence is to develop opioid drugs that selectively target one or a subset of opioid receptors. Another approach is the use of recombinant, e.g., mutant, opioid receptors. For example, Carlezon et al. (1997) overexpressed the AMPA receptor subunit GluR1 in the ventral tegmental area (VTA) using herpes simplex virus-mediated gene transfer. Carlezon et al. found that the expression of GluR1 increased the sensitization of virally-transduced animals to morphine's stimulant and rewarding effects. Kong et al. (1993) substituted aspartic acid 95 in transmembrane segment 2 of the cloned mouse δ opioid receptor with an asparagine (D95N). The D95N mutant receptor had reduced affinity for δ receptor-selective agonists such as enkephalin, [D-Pen2, D-Pen5]enkephalin and [D-Ser2,Leu5]enkephalin-Thr6. The binding of δ-selective non-peptide agonists was also reduced. In contrast, δ receptor-selective antagonists, such as naltrindole, the benzofuran analog of naltrindole, and 7-benyllidenenaltrexone, bound equally well to the wild-type and mutant receptor. Non-selective opioid agonists such as bremazocine and buprenorphine, which interact with δ, κ, and μ opioid receptors, showed no difference in binding to the wild-type and mutant δ receptor.
Claude et al. (1996) investigated the involvement of a conserved serine (Ser196 at the μ, Ser177 at the δ, and Ser187 at the κ opioid receptor) in receptor activation. Claude et al. noted that classical opioid antagonists such as naloxone, naltrexone, naltriben, and H-Try-Tic[ψCH2NH]Phe-Phe-OH (TIPPψ, Tic=1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) could inhibit forskolin-stimulated adenylyl cyclase activity in CHO cells stably expressing a μ/δ opioid chimeric receptor, μδ2. Antagonists also activated the G protein-coupled inward rectifying potassium channel (GIRK1) in Xenopus oocytes coexpressing the μδ2 opioid receptor and the GIRK1 channel. In vitro experiments in which the Ser196 residue of the μ opioid receptor was substituted with other amino acids, indicated that a mutation of Ser196 to Ala resulted in greater agonistic efficacy with classical opioid antagonists (Claude, 1997).
What is needed is an improved method to identify agents that have desirable effects of opioids in the absence of unwanted side effects.